1. Field of the Invention
The present invention relates to a Group III nitride semiconductor device (also referred to as device hereinbelow) and a method for manufacturing the same.
2. Description of the Related Art
Extensive research is now underway on semiconductor light-emitting devices, in particular, a short-wave length semiconductor laser device based on gallium nitride (GaN) and related compounds as a material system for the device. A GaN-based semiconductor laser device is manufactured by successively depositing semiconductor single-crystal layers such as (AlxGa1-x)1-yInyN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61) on a crystal substrate.
A metal organic chemical vapor deposition method (abbreviated as MOCVD hereinbelow) is generally used to produce such a single-crystal layer. In this method, source gases containing trimethyl gallium (abbreviated as TMG hereinbelow) as a Group III precursor material and ammonia (NH3) as a Group V precursor material are introduced into a reactor to react at a temperature within the range of 900-1000xc2x0 C., thereby depositing compound crystals on the substrate. Various compounds can be layered on the substrate by changing the ratio of the precursors fed into the reactor to obtain a multi-layer structure.
If the deposited single-crystal layer has many penetrating defects, the light emitting performance of the device is deteriorated substantially. Such defect is called threading dislocation, which is a linearly extending defect that penetrates the crystal layer along the growth direction. Since a threading dislocation acts as a non-radiative recombination center for carriers, a semiconductor light-emitting device comprising a layer with many dislocations suffers from poor luminous efficiency. The mentioned above defect is generated due to crystal misfit strain at an interface between the substrate and an overlying layer formed thereon. Attempts to reduce the effect of the misfit at the interface have been made by choosing substrate materials having similar crystal structure, lattice constant, and thermal expansion coefficient to those of GaN-based crystal.
There is no substrate low-priced and lattice-matched to a nitride semiconductor. Sapphire plates are therefore mainly utilized as substrates for the epitaxial growth of nitride semiconductor. In this case, the threading dislocations are generated due to the lattice mismatch since sapphire has a lattice constant different from that of GaN by about 14%. It is unavoidable that the density of threading dislocations becomes 1E8/cm2 or more even under the best conditions. Epitaxially Lateral Over-growth (ELO) or the like can drastically reduce the dislocation density. However ELO drastically raises the manufacturing cost of devices. The adaptation of ELO to the manufacture of the nitride semiconductor device such as a light emitting diode or the like has no practical use.
Unexamined Japanese patent KOKAI Publication No. 2000-232238 (U.S. Pat. No. 6,329,667) discloses a conventional technology improving some properties of the nitride semiconductor laser device. In the disclosed prior art, formation of pits or recesses about the threading dislocations is preformed during epitaxial growth on a wafer after the growth of the active layer is finished. Then the pits of the active layer are buried with a material having a wider band-gap than that of the active layer and, after that, the other structural layers of device are layered. This technology improves the luminescence characteristics of the device because injection of carriers to the threading dislocations is avoided.
In the case of a pn junction diode formed by growing nitride semiconductors onto a dissimilar substrate such as a sapphire substrate or the like, the leakage current under reverse bias tends to be high in comparison with that of the semiconductor device of GaAs or the like. This diode characteristic originates in the high density of threading dislocations in the grown layers mentioned above.
The inventors have revealed that, although the prior art mentioned above does permit the decrease of luminescence efficiency caused by the threading dislocations under forward injection to improve the luminescence characteristics of the device, it still fails to solve the reverse leakage current problem. The increase of the reverse leakage current incurs substandard products resulting in hindering the increase of production yield. For example, the technical specification for a light emitting diode usually involves an item of leakage current under an application of reverse voltage thereto, e.g., less than 10 xcexcA at 5 V applied.
The present invention has been made in view of the deterioration of current-voltage characteristic, i.e., the large reverse leakage of the nitride semiconductor device described above, and an object thereof is to provide a nitride semiconductor device having good current-voltage characteristics while allowing the generation of defects passing through the single-crystal layers grown on a substrate.
According to one aspect of the present invention, there is provided a nitride semiconductor device including Group III nitride semiconductors comprising:
an active layer;
a barrier layer made from a predetermined material and provided adjacent to said active layer, said barrier layer having a greater band-gap than that of said active layer;
a barrier portion formed of said predetermined material for surrounding a threading dislocation in said active layer, said barrier portion having a vertex; and
a semiconductor layer having an impurity concentration ranging from 1E16/cc to 1E17/cc in which said vertex is placed.
In the nitride semiconductor device mentioned above, said active layer has one of a single and multiple quantum well structure.
In the nitride semiconductor device mentioned above, said predetermined material of said barrier layer fills up a recess enclosed with an interface on said active layer to smooth surfaces of said recess as the barrier portion.
In the nitride semiconductor device mentioned above, said barrier portion has one of a cone-shape, truncated cone shape and a combination thereof.
In the nitride semiconductor device mentioned above, said Group III nitride semiconductors are (AlxGa1-x)1-yInyN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61).
The nitride semiconductor device mentioned above may further comprise a low temperature barrier layer provided between said barrier layer and said active layer, said low temperature barrier layer being formed of substantially the same predetermined material as that of said barrier layer at substantially the same temperature as the growth temperature of said active layer.
In the nitride semiconductor device mentioned above, said low temperature barrier layer has a lower AlN composition ratio than that of said barrier layer.
According to another aspect of the present invention, there is provided a method for manufacturing a nitride semiconductor device including Group III nitride semiconductors and having an active layer and a barrier layer made from a predetermined material with a greater band-gap than that of the active layer and disposed adjacent to said active layer, the method comprising the steps of:
forming a semiconductor layer having an impurity concentration ranging from 1E16/cc to 1E17/cc;
forming the active layer over the semiconductor layer having a recess attributable to a threading dislocation in the active layer; and
depositing the material of the barrier layer onto the active layer to form a barrier portion surrounding the threading dislocation and having an interface defined by the side surface of the recess.
In the method for manufacturing a nitride semiconductor device mentioned above, the step of forming the active layer includes a step of etching the active layer after the active layer is deposited.
In the method for manufacturing a nitride semiconductor device mentioned above, the etching in the step of etching is terminated when erosion along the threading dislocation reaches the underlying semiconductor layer.
In the method for manufacturing a nitride semiconductor device mentioned above, the step of forming the semiconductor layer at a temperature within a range of 600-850xc2x0 C. prior to the growth of the active layer.
In the method for manufacturing a nitride semiconductor device mentioned above, the method further comprises the step of forming a low temperature barrier layer of substantially the same material as that of the barrier layer at substantially the same temperature as a growth temperature of the active layer between the step of forming the pit and the step of depositing the material.
In the method for manufacturing a nitride semiconductor device mentioned above, the low temperature barrier layer has a lower AlN composition ratio than that of the barrier layer.